1. Field of the Invention
The invention concerns a process for controlling a parametric loudspeaker system, comprised of (a) one or more transducer elements for ultrasound, which can be driven to produce an AM-signal, which during propagation in a gaseous medium produces an audible signal by self demodulation, (b) one or more amplifiers associated with these transducer elements, and (c) one or more modulators associated therewith, which receive an input signal from a signal source, and a device suitable for carrying out the process.
2. Description of the Related Art
An emission of directional sound waves requires a sound transducer with a geometric size in the range of multiple wavelengths. In place of a single transducer it is also possible to employ multiple transducers in order to produce the large geometric measurement. An arrangement of multiple transducers is referred to as an array. The individual transducers can additionally have an upstream signal processor in order to increase the directionality of the array.
In order to produce a strong directionality with small transducer size a modulation technique can be employed in order to couple a low frequency useful signal (audio signal) with a high frequency carrier signal. It is the wavelength of the higher frequency carrier signal that is primarily determinative of directionality. A parameter of the carrier signal is controlled by the useful signal. From this, the term parametric transducer or parametric array is derived.
The present invention is concerned with a parametric loudspeaker which employs ultrasound as the carrier signal. The basic physical experiments can be traced back to the German physicist Helmholz in the 19th century. A useful loudspeaker system is described by Yoneyama, et al.: “The Audio Spotlight: An Application of Nonlinear Interaction of Sound Waves to a new Type of Loudspeaker Design”; J. Acoust. Soc. Am., Vol. 73, pp. 1532–1536. Reports thereof were made in the subsequent years in further publications of Berktay, Blackstock, Pompei and others.
If ultrasound is emitted at very high levels, the air becomes a nonlinear medium, which causes a self-demodulation of the modulated ultrasound on the basis of the nonlinearity. Therewith, the modulated signal becomes audible. The ultrasound itself remains inaudible.
From WO 01/08449 A1 a process for reproducing audio waves using ultrasound loudspeakers is known, wherein the audio signal to be reproduced is coupled with a carrier signal in the ultrasound frequency range by a side-band amplitude modulation. Therein the modulation is either realized as conventional two side band AM or as one side band AM, wherein the carrier is suppressed by approximately 12 dB for further functional optimization. In particular in the employment of transducers with strong nonlinear frequency paths it is herein advantageous to achieve a linearization of the frequency path, in order to balance out frequency dependent amplitude defects.